1. Field of the Invention
This invention relates to electroplating methods and solutions and, more particularly, to methods and electroplating solution chemistries for co-electrodeposition of at least one Group IIIA material and at least one Group VIA material on a conductive surface to form a Group IIIA-Group VIA compound or mixture layer with predetermined composition or stoichiometry.
2. Description of the Related Art
Solar cells are photovoltaic devices that convert sunlight directly into electrical power. The most common solar cell material is silicon, which is in the form of single or polycrystalline wafers. However, the cost of electricity generated using silicon-based solar cells is higher than the cost of electricity generated by the more traditional methods. Therefore, since the early 1970's there has been an effort to reduce the cost of solar cells for terrestrial use. One way of reducing the cost of solar cells is to develop low-cost thin film growth techniques that can deposit solar-cell-quality absorber materials on large area substrates and to fabricate these devices using high-throughput, low-cost methods.
Group IBIIIA VIA compound semiconductors comprising some of the Group IB (Cu, Ag, Au), Group IIIA (B, Al, Ga, In, Tl) and Group VIA (O, S, Se, Te, Po) materials or elements of the periodic table are excellent absorber materials for thin film solar cell structures. Especially, compounds of Cu, In, Ga, Se and S which are generally referred to as CIGS(S), or Cu(In,Ga)(S,Se)2 or CuIn1−xGax(SySe1−y)k, where 0≦x≦1, 0≦y≦1 and k is approximately 2, have already been employed in solar cell structures that yielded conversion efficiencies approaching 20%. Absorbers containing Group IIIA element Al and/or Group VIA element Te also showed promise. Therefore, in summary, compounds containing: i) Cu from Group IB, ii) at least one of In, Ga, and Al from Group IIIA, and iii) at least one of S, Se, and Te from Group VIA, are of great interest for solar cell applications.
The structure of a conventional Group IBIIIA VIA compound photovoltaic cell such as a Cu(In,Ga,Al)(S,Se,Te)2 thin film solar cell is shown in FIG. 1. The device 10 is fabricated on a substrate 11, such as a sheet of glass, a sheet of metal, an insulating foil or web, or a conductive foil or web. The absorber film 12, which comprises a material in the family of Cu(In,Ga,Al)(S,Se,Te)2 is grown over a conductive layer 13, which is previously deposited on the substrate 11 and which acts as the electrical contact to the device. Various conductive layers comprising Mo, Ta, W, Ti, and stainless steel etc. have been used in the solar cell structure of FIG. 1. If the substrate itself is a properly selected conductive material, it is possible not to use a conductive layer 13, since the substrate 11 may then be used as the ohmic contact to the device. After the absorber film 12 is grown, a transparent layer 14 such as a CdS, ZnO or CdS/ZnO stack is formed on the absorber film. Radiation 15 enters the device through the transparent layer 14. Metallic grids (not shown) may also be deposited over the transparent layer 14 to reduce the effective series resistance of the device. A variety of materials, deposited by a variety of methods, can be used to provide the various layers of the device shown in FIG. 1. It should be noted that although the chemical formula for a CIGS(S) layer is often written as Cu(In,Ga)(S,Se)2, a more accurate formula for the compound is Cu(In,Ga)(S,Se)k, where k is typically close to 2 but may not be exactly 2. For simplicity we will continue to use the value of k as 2. It should be further noted that the notation “Cu(X,Y)” in the chemical formula means all chemical compositions of X and Y from (X=0% and Y=100%) to (X=100% and Y=0%). For example, Cu(In,Ga) means all compositions from CuIn to CuGa. Similarly, Cu(In,Ga)(S,Se)2 means the whole family of compounds with Ga/(Ga+In) molar ratio varying from 0 to 1, and Se/(Se+S) molar ratio varying from 0 to 1.
The first technique that yielded high-quality Cu(In,Ga)Se2 films for solar cell fabrication was co-evaporation of Cu, In, Ga and Se onto a heated substrate in a vacuum chamber. Another technique for growing Cu(In,Ga)(S,Se)2 type compound thin films for solar cell applications is a two-stage process where at least two components of the Cu(In,Ga)(S,Se)2 material are first deposited onto a substrate, and then reacted with S and/or Se in a high temperature annealing process. For example, for CuInSe2 growth, thin layers of Cu and In are first deposited on a substrate and then this stacked precursor layer is reacted with Se at elevated temperature. If the reaction atmosphere also contains sulfur, then a CuIn(S,Se)2 layer can be grown. Addition of Ga in the precursor layer, i.e. use of a Cu/In/Ga stacked film precursor, allows the growth of a Cu(In,Ga)(S,Se)2 absorber.
Sputtering and evaporation techniques have been used in prior art approaches to deposit the layers containing the Group IB and Group IIIA components of the precursor stacks. In the case of CuInSe2 growth, for example, Cu and In layers were sequentially sputter-deposited on a substrate and then the stacked film was heated in the presence of gas containing Se at elevated temperature for times typically longer than about 30 minutes, as described in U.S. Pat. No. 4,798,660. More recently U.S. Pat. No. 6,048,442 disclosed a method comprising sputter-depositing a stacked precursor film comprising a Cu—Ga alloy layer and an In layer to form a Cu—Ga/In stack on a metallic back electrode layer and then reacting this precursor stack film with one of Se and S to form the absorber layer. Such techniques may yield good quality absorber layers and efficient solar cells, however, they suffer from the high cost of capital equipment, and relatively slow rate of production.
One prior art method described in U.S. Pat. No. 4,581,108 utilizes a low cost electrodeposition approach for metallic precursor preparation. In this method a Cu layer is first electrodeposited on a substrate. This is then followed by electrodeposition of an In layer and heating of the deposited Cu/In stack in a reactive atmosphere containing Se. While substantially pure metallic stacks may be used as precursors to form CIGS type solar cell absorbers, it is also possible to alloy Se and/or S with In to form precursor stacks containing In containing compound phases such as InSe, InS, In2Se3 and In2S3. Similarly, Ga-selenide or sulfide layers may also be included in the precursor stacks. By including compound phases such as (In,Ga)—(S,Se) into the precursor stacks the reaction kinetics and thus the morphology and micro-structure of the resulting CIGS layers may be affected. It should be noted that (In,Ga)—(S,Se) film means a layer with a composition of InxGaySzSem, where x, y, z and m can take any value with the conditions that if x is 0, y cannot be 0 and vice versa, and if z is zero m cannot be zero and vice versa.
In—Se mixture or alloy layers have previously been deposited using electroplating techniques. Hirano, for example, electrodeposited In layers out of acidic solutions that contained a suspension of fine Se particles (U.S. Pat. No. 5,489,372). As described by Hirano, this method yielded an electrodeposited In layer which contained dispersed selenium particles since during electrodeposition of In the Se particles near the surface of the cathode got trapped in the growing layer. As can be appreciated, this method forms a composite layer comprising electroplated In and mechanically trapped Se particles. Selenium deposition does not involve any electrochemical reduction reaction on the cathode surface, and thus the technique is not expected to be very repeatable for electronic device fabrication applications.
Electrochemical co-deposition of In and Se on a cathode surface has also been demonstrated in some prior art work. For example, Igasaki et al. (J. Crystal Growth, 1996, vol. 158, p. 268) electroplated In—Se material out of an electrolyte containing hydrochloric acid which was used to adjust the pH value of the solution to a range of 1.0-1.7. Massaccesi et al., on the other hand, used another acidic solution based on sulfuric acid (J. Electroanal. Chem., 1996, vol. 412, p. 95). This solution had a pH value of 3.45. Fernandez et al. (WREC 1996, p. 396) electroplated In—Se layers using acidic baths comprising In ions and H2SeO3. Kampmann et al. (Progress in PV, 1999, vol. 7, p. 129; and, Thin Solid Films, 2000, vol. 361, p. 309) reported In2Se3 film electrodeposition out of an acidic electrolyte with a pH value of 2.4. Fernandez et al. (Advanced Materials for Optics and Electronics, 1998, vol. 8, p. 1; and, Solar Energy Materials and Solar Cells, 1998, vol. 52, p. 423) and Hermann et al. (Solar Energy Materials and Solar Cells, 1998, vol. 52, p. 355) employed an In2Se3 plating electrolyte with a pH of 1.5. All of the above listed electrochemical co-deposition approaches utilize simple acidic electrolytes with pH values less than 7 and they aim to obtain In—Se alloy layers through the cathodic reaction (1) given below.xSe(IV)+yIn(III)+(4x+3y)e−→InySex  (1)
The above mentioned prior art acidic plating baths do not yield stable and repeatable electrodeposition process and high quality films that can be used in electronic device applications. Therefore, the present invention aims to develop an efficient electroplating bath to deposit smooth and defect-free Group IIIA-Group VIA alloy or mixture films in repeatable manner, where the Group IIIA material is at least one of In and Ga and the Group VIA material is at least one of Se, Te and S.